Many commercially important therapeutic proteins, including erythropoietin (EPO), tissue plasminogen activator (t-PA), urokinase and prourokinase, growth hormones, cytokines, factor VIII, epoetin-.alpha., granulocyte colony stimulating factor, and vaccines, are produced by recombinant methods, typically by microbial fermentation. More recently, methods using mammalian cell culture or transgenic animals have been employed in producing some therapeutic proteins, and have the advantage of producing protein glycosylation that may confer advantages in terms of increased stability or activity of the proteins.
Once a protein has been approved for human therapeutic use, it is generally desirable, where alternative methods of synthesis are sought, to alter the synthesized protein as little as possible. Thus, for example, if the approved protein has a known sequence with given N- and C-terminal residues, it is desirable that any new method of protein synthesis be capable of producing a protein whose sequence is identical to that of the already approved mature protein.